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Researchers develop energy-efficient window coating

UC Berkeley and Lawrence Berkeley National Laboratory researchers developed a coating material for windows that can control the amount of heat from the sun that enters a room, setting the groundwork to save energy in large commercial buildings and homes.

The material — made of semiconductor nanocrystals — has the ability to control the amount of heat from the sun that passes through it while not changing the visible light in the room. The coating is the first such material that researchers have developed that keeps the window transparent without any tint or cloudiness, according to a study published Aug. 25 in the journal Nano Letters, an American Chemical Society publication.

“The distinguishing feature of our new coating is that the ‘tinting’ isn’t visible by eye — only the amount of heat transmitted by the glass changes, not the light that you can see (and use to light your building),” said Delia Milliron, a staff scientist at Berkeley Lab and co-author of the study, in an email.

The new coating material could save on energy costs by controlling the sun’s heat depending on the temperature of the season, she said.

Windows installed in commercial buildings often block near-infrared wavelength light, which only contributes heat, Milliron said. However, though this technique reduces the amount of heat in the room, it does not allow heat to be let in when desired.

The researchers plan to develop a multilayer window that would include two layers of the semiconductor nanocrystal coating, which would allow control of the amount of near infrared light — and therefore heat — Milliron said.

According to Guillermo Garcia, a graduate student in the UC Berkeley Department of Mechanical Engineering and the lead author of the study, the semiconductor nanocrystals used in the coating are a common material for conducting electricity, but they have never been used in window coatings before.

“The reason we can absorb the heat from the sun is the fact that it is electrically conductive,” Garcia said. “It has to have a lot of free electrons in the material, and when that vibrates, it absorbs a region of light.”

Garcia said the study successfully demonstrated a proof of concept with the new coating material. Though they are still in the very early stages of development, researchers are eventually hoping to scale up the project to make it feasible on a commercial scale, where the greatest energy savings would take place in homes, Milliron said.

“In the summer, these windows could be used to block as much heat from the sun as possible to keep the room cool, while in the winter, allow as much sun heat without using the boiler,” Garcia said.

If we could manipulate this material to generate electricity while controlling heat gain through a window, we’d have a real winner.

Guest

But much of the problem with solar heating is that light is absorbed by objects in the room and re-radiated as heat. If infra-red alone were the cause, a thin film of water would block it.

Tim

Yes. In fact, the actual “greenhouse effect” occurs because the re-radiated heat can’t pass through the glass and is trapped inside. The amount of infra-red passing through the glass from outside must be small.